The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface can present problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, plaranization is needed when polishing back a filler layer, e.g., when filling trenches in a dielectric layer with metal.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened or soft surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-active agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency of the substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in non-uniform polishing at the substrate perimeter, e.g., the outermost three to fifteen millimeters of a 200 millimeter (mm) wafer.
In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure and a first flexible membrane. The first flexible membrane has central portion and an annular portion surrounding the central portion. A volume between the first flexible membrane and the carrier structure provides a first pressurizable chamber. The central portion of the first flexible membrane is formed of a first material with a different rigidity than a second material that forms the annular portion of the first flexible membrane.
Implementations of the invention may include one or more of the following features. The first material may be less rigid than the second material. The first and second materials may be elastomers. The annular portion may be a perimeter portion of the first flexible membrane secured to the carrier structure. The perimeter portion may be secured between the carrier structure and a retaining ring. The first flexible membrane may include an annular connector portion extending between the central portion and the perimeter portion. The connector portion may be formed of a third material that has is more rigid than the first material and less rigid than the second material. The first and third materials may be elastomers, and the second material may be a fiber-reinforced elastomer. A second flexible membrane may be secured to the carrier structure and may extend below the first flexible membrane. A lower surface of the second flexible membrane may provide a substrate mounting surface. The annular portion may have a convolution.
In another aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier structure and a first flexible membrane. The first flexible membrane has a perimeter portion secured to the carrier structure, a central portion positioned to apply a pressure to a substrate, and a connector portion extending between the central portion and the perimeter portion. A volume between the first flexible membrane and the carrier structure provides a first pressurizable chamber. The perimeter portion of the first flexible membrane is formed of a first material, the connector portion is formed of a second material that is more rigid than the first material, and the central portion is formed of a third material that is more rigid than the second material.
Implementations of the invention may include one or more of the following features. The first and second materials may be elastomers, and the third material may be a fiber-reinforced elastomer. An annular flap may be joined to the connector portion, and an edge of the flap may be secured to the carrier structure. The flap may be formed of the first material.
In another aspect, the invention is directed to a flexible membrane for a carrier head. The flexible membrane has a central portion formed of a first material and a perimeter portion formed of a second material having a different rigidity than the first material.
Implementations of the invention may include one or more of the following features. The first material may be more rigid than the second material. The central portion may apply a pressure to a substrate, and the perimeter portion may be secured to a carrier structure.
In another aspect, the invention is directed to a flexible membrane for a carrier head that has a perimeter portion, a central portion, and a connector portion extending between the central portion and the perimeter portion. The perimeter portion of the flexible membrane is formed of a first material, the connector portion is formed of a second material that is more rigid than the first material, and the central portion is formed of a third material that is more rigid than the second material.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing. The carrier head has a carrier structure, a first flexible membrane connected to the carrier structure, and a second flexible membrane connected to the carrier structure. A first volume between the carrier structure and the first flexible membrane provides a first chamber, and a second volume between the first flexible membrane and the second flexible membrane providing a second chamber. The second flexible membrane has a lower surface that provides a mounting surface for a substrate. At least one of a top surface of the second flexible membrane and a bottom surface of the first flexible membrane is textured to prevent adhesion between the first flexible membrane and the flexible membranes when they contact.
Potential advantages of implementations of the invention may include zero or more of the following. The distribution of pressure at the substrate edge may be controlled. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. Non-uniform polishing of the substrate is reduced, and the resulting flatness and finish of the substrate are improved.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.